1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus of an electrophotographic type, such as a copying machine, a facsimile apparatus, a printer and a complex machine (hereinafter referred simply to “image forming apparatus”).
2. Description of the Related Art
With an image forming apparatus in which an electrostatic latent image is formed on an electronic photosensitive drum (hereinafter referred to as “photosensitive drum”) as an image bearing member and is developed using toner, a toner image is transferred on a sheet, such as recording paper, being loaded on, for example, an endless conveyance belt. Alternatively, an intermediate transfer method is available which uses a conveyance belt thereof as an intermediate transfer belt, once transfers a toner image and holds it, and then shifts the toner image to a sheet so as to transfer the toner image.
In this case, if a relative speed difference is caused between a speed of circumference of a photosensitive drum (circumferential speed) and a traveling speed of the conveyance belt, toner images are affected resulting in a decrease in image quality or deterioration of image quality after printing. As a method to eliminate a relative speed difference as mentioned, such a configuration is available in which, a torque limiter is provided to a driving portion of the transfer belt, when a slippage is caused between the photosensitive drum and the transfer belt, drum speed is caused to follow circumferential speed of the transfer belt to reduce the slippage between the transfer belt and drum (see, Japanese Patent No. 2971615). However, this configuration is designed to follow the photosensitive drum at an abnormality of a slippage occurrence.
In the meantime, such a structure is known that, for the sake of attaining high image quality, a traveling speed of the transfer belt is maintained constant, the photosensitive drum is contacted against a belt plane thereof to allow rotation dependent on frictional force. Alternatively, a drum-dedicated belt which is provided separately is directly wound around the photosensitive drum or a pulley on the same axis, the drum-dedicated belt is revolved by a driving motor commonly used for revolving the conveyance belt (transfer member belt). Namely, the image forming apparatus proposed is designed such that one driving motor is used for revolving both the drum-dedicated belt and the conveyance belt so that a relative speed difference may not be caused between the photosensitive drum and the conveyance belt (Japanese Patent Application Laid-Open No. 11-24350).
However, the drum driving structure as disclosed in Japanese Patent No. 2971615 and conventional structures of this sort have the following problems:
One problem is that, in order to prevent a speed difference due to a slippage between photosensitive drum and the conveyance belt, it is necessary to tightly contact the photosensitive drum and the conveyance belt mutually by a strong pressing force. As a result, a problem arises in that load of the driving motor becomes excessive.
Another problem is that, the lifetimes of the drum and belt are shortened due to stresses caused by strong pressure.
From the above, an object of the present invention is to provide an image forming apparatus capable of forming good images while a belt and an image bearing member are made to be driven without increasing a pressing force between an endless transfer member belt and an image bearing member.